1. Field of the Invention
This invention relates to an optical fiber bundle, and more particularly to an improvement in the structure of an end face of an image transmitting optical fiber bundle (hereinafter referred to simply as "image guide").
2. Description of the Prior Art
In the image guide composed of a number of thin optical fibers stacked in a close-packed structure, the outer fibers on the periphery of the image guide are liable to slack and the arrangement of the fibers is apt to be disordered. Particularly, in image guides which are combined with an optical system used for enlarging an image, the disordered arrangement of the fibers is also enlarged, which results in degradation in image quality. In order to evade this problem, it has been generally known to provide a field stop between the end of the image guide and an eyepiece.
In the image guide practically manufactured, the end face of the image guide is precise finished to make an optically flat end face. In order to prevent the finished end from staining, weathering or dimming, a face plate is generally cemented to the finished end of the image guide. Therefore, in the conventional image guide, the face plate is fixed directly to the end of the image guide and thereon is mounted a field stop to provide a mask on the image obtained by the image guide. An eyepiece is then provided on the field stop.
The above described conventional image guide has a defect in that the effective aperture size of the field stop is changed as the position of the pupil is moved along the optical axis of the eyepiece, since the field stop is separated from the end face of the image guide. When a picture of the image viewed through the image guide is taken, the marginal boundary of the image defined by the field stop is apt to be blurred.
If the field stop would be inserted between the eyepiece and the end face of the image guide in order to preclude the above described defect caused by the distance made therebetween, there would occur the following problems. When the field stop is fixed to the end face of the image guide, an adhesive is used. In this case, it is very difficult to remove bubbles contained in the adhesive. The bubbles in the adhesive will accelerate the staining, weathering or dimming of the end face of the image guide. Since the inner edge of the field stop which has a substantial thickness works to accelerate the formation of the bubbles when the field stop is pressed against the end face of the image guide with the adhesive sandwiched therebetween, the removal of the bubbles is considerably difficult. When the bubbles are fixedly formed in the adhesive, it would become necessary to remove the field stop from the end face to again polish the end face.